When aromatic compounds such as those defined hereinbefore are oxidized with nitric acid, the oxidation is generally conducted in vertical reactor columns.
In such oxidation reactions, it is necessary that the nitric acid and aromatic compound be intimately mixed and that, after passing through the column, the oxidation has progressed either to the final stage or to a stage at which intermediate products which are soluble in hot nitric acid are obtained. This applies especially to that portion of the aromatic compound that is present in the form of vapor in the bubbles of the waste gases and is carried along with such bubbles at a high speed upwardly in the reactor column.
A continuous process is described in U.S. Pat. No. 3,165,548 and German Patent No. 1,119,842 for the production of aromatic carboxylic acids by oxidizing a compound having an aromatic nucleus and an oxidizable side chain or acyclic substituent with nitric acid containing between 10 and 40% by weight of nitric acid at a temperature between 150.degree. and 400.degree.C and a superatmospheric pressure between 10 and 150 atmospheres in which the aromatic compound that is to be oxidized is premixed with the nitric acid at a temperature below the oxidation temperature and the mixture is then passed in turbulent flow into the lower end of a vertical elongated reaction zone filled with the reaction mixture and maintained adiabatically at the said oxidation temperature and pressure, the velocity of flow of the said reaction mixture decreasing to at least one-third of its original rate upon entry into the said reaction zone so as to form a vortex with the hot reaction product that is sucked back to the point of entry of the said reaction mixture, while avoiding a gas space at the upper end of the said reaction zone, and withdrawing the resulting aromatic carboxylic acid at the upper end of the said reaction zone.
An improvement of the foregoing process is described in U.S. Pat. No. 3,271,445 and German Patent No. 1,443,435 in which between 30 and 60% by weight of the nitric acid that is to be used is introduced into the reaction zone from the bottom or lower end of the reaction zone and between 40 and 60% by weight into the middle third of the reaction zone.
However, it is to be noted that the process is described in these patents as being carried out in a tantalum-lined reactor. Furthermore, the yields of aromatic carboxylic acid are unsatisfactory and the losses of nitric acid are unusually high.
In German Patent No. 1,268,131 and United Kingdom Patent No. 1,149,321 is described a process for the continuous production of trimellitic acid by oxidation of 1,2,4-trimethylbenzene (pseudocumene) in which the 1,2,4-trimethylbenzene in the form of mechanically produced globules or droplets is introduced upwardly into the lower end of a pressurized vertical reactor column containing nitric acid having a concentration between approximately 4 and approximately 14% by weight of nitric acid that had been heated to a temperature between 160.degree. and 180.degree.C. The reaction mixture after passing through the column is passed also upwardly from its lower end into a subsequent reactor column containing nitric acid in which the oxidation is carried to completion at a temperature between 200.degree. and 210.degree.C. The waste gases that are formed in both columns are each withdrawn at the tops of each column. In this process, concentrated nitric acid is introduced into the reactor columns to increase the concentration of the nitric acid in the reaction mixture that is flowing therethrough.
It has however been found that, in order to scale-up the size of the columns for commercial production, it is not practical to simply increase the dimensions of a pilot model correspondingly. Another disadvantage of the process that is described in German Patent No. 1,268,131 is that, when the aromatic compound that is to be oxidized is brought into contact with an inadequate quantity of dilute nitric acid, even in the first stage of the oxidation, a momentary deficiency of the oxidizing agent is created. This deficiency cannot be compensated for in an ideal manner by supplying more-concentrated nitric acid to the principal reaction zone, and it is not possible to locate the regions in the columns at which such momentary demands for oxidizing agent occur at the moment they occur. The nitric acid concentration is accordingly directly subject to constant fluctuations, as a result of which the oxidation product is obtained in lower yield than is expected. Furthermore, the oxidation reaction may proceed in competition with undesirable nitration reactions of the aromatic nucleus. 1,2,4-Trimethylbenzene, for example, is susceptible to such nitration reactions. substantial losses of yield of the desired aromatic carboxylic acid result from such nitration reactions and nitric acid is needlessly consumed in such reactions.